Metcal, Inc., of Menlo Park, Calif. has developed temperature self-regulating soldering iron systems in which an alloy heater element is disposed within a magnetic solenoidal coil. This structure is know as “solenoidal coupling”. An advantage of this design is that it is very magnetically efficient. Examples of such solenoidal coupling systems are found in U.S. Pat. Nos. 4,745,264 and 4,839,501, both assigned to Metcal, Inc.
Unfortunately, although solenoidal coupling designs are very magnetically efficient, they are not as thermally efficient as could be desired. This is due to the comparatively small diameter (and thus small cross sectional area) of the heater element that is received within the surrounding magnetic solenoidal coil. Such small diameter heater elements have a comparatively high thermal resistance (i.e. low thermal efficiency) due to their small cross sectional area (through which the heat is axially conducted).
Such thermal inefficiencies are especially pronounced when the heater element of the soldering iron is temperature “self-regulating”. This is due to the fact that such temperature self-regulating heater elements typically comprise an inner copper core (which conducts heat well, but which only conducts current therethrough when the copper reaches its Currie Temperature) and an outer alloy heating layer (in which the heat is generated). At low temperatures, current primarily passes through the outer alloy layer of the heater element. The inner copper core acts as the principal thermal conductor. Therefore, it is desirable to maintain a sufficiently large copper core thickness (and thus a sufficiently large cross-sectional area) to maintain a sufficiently high overall thermal efficiency for the heater element. A large diameter copper core unfortunately results in the tip of the soldering iron having a large base diameter. It would instead be desirable to keep the diameter of the base of the tip small (to facilitate viewing during soldering operations).
To compensate for the low thermal efficiencies of such solenoidal coupling designs, a high frequency power supply is therefore typically required. This is due to the fact that the amount of power generated by the heater element is a function of the surface area of the alloy heating layer times the watt density passing therethrough. Since the heating layer has a comparatively small diameter (to fit within the surrounding magnetic coil) the heating layer will also have a comparatively small surface area. A small surface area alloy heating layer will therefore require a higher watt density heater. Consequently, a higher operating frequency power supply will be required to generate this required increased watt density. Unfortunately, such high frequency power supplies tend to be expensive.
As stated above, the thermal inefficiencies of solenoidal coupling designs become especially pronounced as the diameter of the base of the tip of the soldering iron is designed to be made smaller and smaller. In view of the above discussed limitations, it is therefore difficult to design a thermally efficient small diameter tip soldering iron in which the magnetic coil wraps around the heater element. This is especially true in the case of temperature self-regulating heater assemblies. In addition, an outer sleeve of ferromagnetic material is required as a shield to minimize coupling in low resistance materials and to prevent radiated emissions.
An alternate design is to instead have the heater element disposed around the magnetic coil. An example of this system can be seen in U.S. Pat. No. 4,877,944, also assigned to Metcal, Inc. An advantage of this design is that it is comparatively more thermally efficient. This is due to the comparatively larger cross sectional area of the heater element (as compared to the above described solenoidal coupling designs). As such, an advantage of this design is that it can be made small enough to fit into a small diameter tip soldering iron. Moreover, when using a temperature self-regulating heater element in this design, the copper layer is instead disposed around the alloy heater (i.e. the opposite of the above described solenoidal coupling designs). Accordingly, the outer copper layer has a larger cross sectional area (as compared to the smaller copper core found in the above described solenoidal coupling designs). Such a larger cross sectional area of the copper layer increases the overall thermal efficiency of the device.
Unfortunately, this design is comparatively less magnetically efficient. This is due to the fact that the magnetic field density is lower on the outside of the magnetic coil (i.e. where the heater element is disposed) than on the inside of the magnetic coil (i.e. where the heater element is disposed on the above described solenoidal coupling design). Accordingly, a higher frequency power supply is typically required to achieve the desired watt densities.
Another problem with soldering irons in general is that their tips are prone to wear out over time, requiring replacement. An example of a replaceable tip system is found in the solenoidal coupling design found in U.S. Pat. No. 5,329,085, also assigned to Metcal, Inc. In this system, the magnetic coil is wrapped around the heater element, and both the heater element and the magnetic coil are part of the cartridge or shaft into which the replaceable tip is received.
An advantage of the '085 system is that it uses a low frequency (i.e. low cost) power supply. Unfortunately, however, the diameter of the heater element is made relatively large to accommodate a thick large cross sectional area copper core so that such a low frequency power supply can be used. In addition, an outer sleeve of ferromagnetic material is required as a shield to minimize coupling in low resistance materials and to prevent radiated emissions.
In view of the forgoing limitations found in the prior art, what is instead desired is a temperature self-regulating soldering iron having a replaceable tip with a small base diameter that can be operated with a low frequency power supply.